Evidence for evolution

Eolio i mmy

Evolution ours when heritale harateristis

of a speies hange.

There is strong evidence for characteristics of species changing over

time. Biologists call this process evolution. It lies at the heart of a

scientic understanding of the natural world. An important distinction

should be drawn between acquired characteristics that develop during

the lifetime of an individual and heritable characteristics that are

passed from parent to offspring. Evolution only concerns heritable

characteristics.

The mechanism of evolution is now well understood – it is natural

selection. Despite the robustness of evidence for evolution by natural

selection, there is still widespread disbelief among some religious

groups. There are stronger objections to the concept that species can

evolve than to the logic of the mechanism that inevitably causes

evolution. It is therefore important to look at the evidence for

evolution.

Eiece fom foil

The fossil reord provides evidene for evolution.

In the rst half of the 19th century, the sequence in which layers

or strata of rock were deposited was worked out and the geological

eras were named. It became obvious that the fossils found in the

various layers were different – there was a sequence of fossils. In the

20th century, reliable methods of radioisotope dating revealed the

ages of the rock strata and of the fossils in them. There has been a

huge amount of research into fossils, which is the branch of science

called palaeontology. It has given us strong evidence that evolution

has occurred.

● The sequence in which fossils appear matches the sequence in which

they would be expected to evolve, with bacteria and simple algae

appearing rst, fungi and worms later and land vertebrates later still.

Among the vertebrates, bony sh appeared about 420 million years

ago (mya), amphibians 340 mya, reptiles 320 mya, birds 250 mya

and placental mammals 110 mya.

● The sequence also ts in with the ecology of the groups, with

plant fossils appearing before animal, plants on land before

animals on land, and plants suitable for insect pollination before

insect pollinators.

● Many sequences of fossils are known, which link together existing

organisms with their likely ancestors. For example, horses, asses

and zebras, members of the genus Equus, are most closely related to

rhinoceroses and tapirs. An extensive sequence of fossils, extending

back over 60 million years, links them to Hyracotherium, an animal

very similar to a rhinoceros.

▲ Figure 1 Fossils of dinosaurs show there were

animals on Earth in the past that had dierent

characteristics from those alive today

▲ Figure 2 Many trilobite species evolved over

hundreds of millions of years but the group is

now totally extinct

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5 E volution and biodi vErsity

Eiece fom elecie beeig

Seletive reeding of domestiated animals shows that

artiial seletion an ause evolution.

Humans have deliberately bred and used particular animal species for

thousands of years. If modern breeds of livestock are compared with

the wild species that they most resemble, the differences are often huge.

Consider the differences between modern egg-laying hens and the

junglefowl of Southern Asia, or between Belgian Blue cattle and the aurochs

of Western Asia. There are also many different breeds of sheep, cattle and

other domesticated livestock, with much variation between breeds.

It is clear that domesticated breeds have not always existed in their

current form. The only credible explanation is that the change has been

achieved simply by repeatedly selecting for and breeding the individuals

most suited to human uses. This process is called articial selection.

The effectiveness of articial selection is shown by the considerable changes

that have occurred in domesticated animals over periods of time that are

very short, in comparison to geological time. It shows that selection can

cause evolution, but it does not prove that evolution of species has actually

occurred naturally, or that the mechanism for evolution is natural selection.

Daa-baed qe: Missing links

An objection to fossil evidence for evolution has

been gaps in the record, called missing links,

for example a link between reptiles and birds.

The discovery of fossils that ll in these gaps is

particularly exciting for biologists.

1 Calculate the length of Dilong paradoxus,

from its head to the tip of its tail. [2]

2 Deduce three similarities between Dilong

paradoxus and reptiles that live on

Earth today. [3]

3 Suggest a function for the protofeathers of

Dilong paradoxus. [1]

4 Suggest two features which Dilong paradoxus

would have had to evolve to become

capable of ight. [2]

5 Explain why it is not possible to be certain

whether the protofeathers of Dilong paradoxus

are homologous with the feathers of birds. [2]

▲ Figure 3 Drawings of fossils recently found in Western

China.They show Dilong paradoxus, a 130-million-year-old

tyrannosauroid dinosaur with protofeathers. a–d: bones of

skull; e–f: teeth; g: tail vertebrae with protofeathers; h–j:

limb bones

(a) (b)

(c)

(d)

(e) (f)

(g)

(h)

(i)

(j)

100 mm

▲ Figure 4 Over the last 15,000 years many breeds of dog have been developed by articial

selection from domesticated wolves

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5.1 E v iDEncE for E volut ion

Daa-baed qe: Domestication of corn

A wild grass called teosinte that grows in Central America was

probably the ancestor of cultivated corn, Zea mays. When teosinte

is grown as a crop, it gives yields of about 150 kg per hectare. This

compares with a world average yield of corn of 4,100 kg per hectare

at the start of the 21st century. Table 1 gives the lengths of some cobs.

Corn was domesticated at least 7,000 years ago.

1 Calculate the percentage difference in length between teosinte

and Silver Queen. [2]

2 Calculate the percentage difference in yield between teosinte

and world average yields of corn. [2]

3 Suggest factors apart from cob length, selected for by farmers. [3]

4 Explain why improvement slows down over generations of

selection. [3]

c aey ad g legh  b (mm)

Teosinte – wild relative of orn 14

Early primitive orn from Colomia 45

Peruvian anient orn from 500 bc 65

Imriado – primitive orn from Colomia 90

Silver Queen – modern sweetorn 170

▲ Table 1

▲ Figure 5 Corn cobs

Eiece fom homologo ce

Evolution of homologous strutures y adaptive

radiation explains similarities in struture when there are

dierenes in funtion.

Darwin pointed out in The Origin of Species that some similarities in

structure between organisms are supercial, for example between a

dugong and a whale, or between a whale and a sh. Similarities like

those between the tail ns of whales and shes are known as analogous

structures. When we study them closely we nd that these structures

are very different. An evolutionary interpretation is that they have had

Homology 

eolio

Looking for patterns, trends

and disrepanies: there are

ommon features in the one

struture of verterate lims

despite their varied use.

Vertebrate limbs are used in

many different ways, such as

walking, running, jumping, ying,

swimming, grasping and digging.

These varied uses require joints that

articulate in different ways, different

velocities of movement and also

different amounts of force. It would

be reasonable to expect them to

have very different bone structure,

but there are in fact common

features of bone structure that are

found in all vertebrate limbs.

Patterns like this require

explanation. The only reasonable

explanation so far proposed in this

case is evolution from a common

ancestor. As a consequence,

the common bone structure of

vertebrate limbs has become a classic

piece of evidence for evolution.

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5 E volution and biodi vErsity

different origins and have become similar because they perform the

same or a similar function. This is called convergent evolution.

Homologous structures are the converse of this. They are structures that

may look supercially different and perform a different function, but

which have what Darwin called a “unity of type”. He gave the example

of the forelimbs of a human, mole, horse, porpoise and bat and asked

what could be more curious than to nd that they “include the same

bones, in the same relative positions”, despite on the surface appearing

completely different. The evolutionary explanation is that they have

had the same origin, from an ancestor that had a pentadactyl or ve-

digit limb, and that they have become different because they perform

different functions. This is called adaptive radiation.

There are many examples of homologous structures. They do not prove

that organisms have evolved or had common ancestry and do not reveal

anything about the mechanism of evolution, but they are difcult to

explain without evolution. Particularly interesting are the structures that

Darwin called “rudimentary organs” – reduced structures that serve no

function. They are now called vestigial organs and examples of them are

the beginnings of teeth found in embryo baleen whales, despite adults

being toothless, the small pelvis and thigh bone found in the body wall

of whales and some snakes, and of course the appendix in humans.

These structures are easily explained by evolution as structures that no

longer have a function and so are being gradually lost.

Pecyl limb

Comparison of the pentadatyl lim of mammals, irds, amphiians and reptiles

with dierent methods of loomotion.

The pentadactyl limb consists of these structures:

Be e femb Hdmb

single one in the

proximal part

humerus femur

two ones in the

distal part

radius and ulna tiia and ula

group of wrist/

ankle ones

arpals tarsals

series of ones in

eah of ve digits

metaarpals and

phalanges

metatarsals

and phalanges

The pattern of bones or a modication of it is

present in all amphibians, reptiles, birds and

mammals, whatever the function of their limbs.

The photos in gure 6 show the skeletons of

one example of each of the four vertebrates

classes that have limbs: amphibians, reptiles,

birds and mammals. Each of them has

pentadactyl limbs:

● crocodiles walk or crawl on land and use their

webbed hind limbs for swimming

● penguins use their hind limbs for walking and

their forelimbs as ippers for swimming

● echidnas use all four limbs for walking and

also use their forelimbs for digging

● frogs use all four limbs for walking and their

hindlimbs for jumping.

Differences can be seen in the relative lengths and

thicknesses of the bones. Some metacarpals and

phalanges have been lost during the evolution of

the penguin’s forelimb.

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5.1 E v iDEncE for E volut ion

speciio

Populations of a speies an gradually diverge into

separate speies y evolution.

If two populations of a species become separated so that they do

not interbreed and natural selection then acts differently on the two

populations, they will evolve in different ways. The characteristics of

the two populations will gradually diverge. After a time they will be

recognizably different. If the populations subsequently merge and have

the chance of interbreeding, but do not actually interbreed, it would be

clear that they have evolved into separate species. This process is called

speciation.

Speciation often occurs after a population of a species extends its range

by migrating to an island. This explains the large numbers of endemic

species on islands. An endemic species is one that is found only in a

certain geographical area. The lava lizards of the Galápagos Islands

are an example of this. One species is present on all the main islands

of the archipelago. On six smaller islands there is a closely related but

different species, formed by migration to the island and by subsequent

divergence.

Ay

Peaday mb 

mamma

porpoise

human

mole

horse

bat

▲ Figure 7 Pentadactyl limbs

(not to scale)

Choose a olour ode for

the types of one in a

pentadatyl lim and olour

the diagrams in gure 7 to

show the type of eah one.

How is eah lim used?

What features of the ones

in eah lim make them well

adapted to the use?

▲ Figure 6

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5 E volution and biodi vErsity

Eiece fom pe of iio

Continuous variation aross the geographial

range of related populations mathes the

onept of gradual divergene.

If populations gradually diverge over time to become separate

species, then at any one moment we would expect to be able

to nd examples of all stages of divergence. This is indeed

what we nd in nature, as Charles Darwin describes in

Chapter II of The Origin of Species. He wrote:

Many years ago, when comparing, and seeing others compare,

the birds from the separate islands of the Galápagos Archipelago,

both one with another, and with those from the American

mainland, I was much struck how entirely vague and arbitrary

is the distinction between species and varieties.

Darwin gave examples of populations that are recognizably

different, but not to the extent that they are clearly separate

species. One of his examples is the red grouse of Britain and the willow

ptarmigan of Norway. They have sometimes been classied as separate

species and sometimes as varieties of the species Lagopus lagopus. This is a

common problem for biologists who name and classify living organisms.

Because species can gradually diverge over long periods of time and

there is no sudden switch from being two populations of one species to

being two separate species, the decision to lump populations together or

split them into separate species remains rather arbitrary.

The continuous range in variation between populations does not match

either the belief that species were created as distinct types of organism

and therefore should be constant across their geographic range or that

species are unchanging. Instead it provides evidence for the evolution of

species and the origin of new species by evolution.

T.albemarlensis

T.duncanensis

T.habelii T.grayii

T.bivittatus

key

Pinta

Marchena

Genovesa

Santiago

San Cristóbal

Santa Fe

Santa Cruz

Santa Maria

Española

Isabela

Fernandina

T.pacicus

T.delanonis

▲ Figure 8 Distribution of lava lizards in the

Galápagos Islands

TOK

t wha exe a mpe mde

be ed  e hee?

The usefulness of a theory is

the degree to whih it explains

phenomenon and the degree to

whih it allows preditions to e

made. One way to test the theory

of evolution y natural seletion is

through the use of omputer models.

The Blind Watchmaker omputer

model is used to demonstrate how

omplexity an evolve from simple

forms through artiial seletion.The

Weasel omputer model is used to

demonstrate how artiial seletion

an inrease the pae of evolution

over random events. What features

would a omputer model have to

inlude for it to simulate evolution y

natural seletion realistially?

Iil melim

Development of melanisti insets in polluted areas.

Dark varieties of typically light-coloured insects are called melanistic.

The most famous example of an insect with a melanistic variety

is Biston betularia, the peppered moth. It has been widely used as

an example of natural selection, as the melanistic variety became

commoner in polluted industrial areas where it is better camouaged

than the pale peppered variety. A simple explanation of industrial

melanism is this:

● Adult Biston betularia moths y at night to try to nd a mate

and reproduce.

● During the day they roost on the branches of trees.

● Birds and other animals that hunt in daylight predate moths if

they nd them.

247

5.1 E v iDEncE for E volut ion

● In unpolluted areas tree branches are covered in pale-coloured

lichens and peppered moths are well camouaged against them.

● Sulphur dioxide pollution kills lichens. Soot from coal burning

blackens tree branches.

● Melanic moths are well camouaged against dark tree branches in

polluted areas.

● In polluted areas the melanic variety of Biston betularia replaced

the peppered variety over a relatively short time, but not in non-

polluted areas.

▲ Figure 10 The ladybug Adalia bipunctata

has a melanic form which has become

common in polluted areas. A melanic male

is mating with a normal female here

▲ Figure 9 Museum specimen of the

peppered form of Biston betularia

mounted on tree bark with lichens

from an unpolluted area

Biologists have used industrial melanism as a classic example of

evolution by natural selection. Perhaps because of this, research

ndings have been repeatedly attacked. The design of some early

experiments into camouage and predation of the moths has been

criticized and this has been used to cast doubt over whether natural

selection ever actually occurs.

Michael Majerus gives a careful evaluation of evidence about the

development of melanism in Biston betularia and other species of moth

in his book in the New Naturalist series (Moths, Michael Majerus,

HarperCollins 2002). His nding is that the evidence for industrial

pollution causing melanism in Biston betularia and other species of moth is

strong, though factors other than camouage can also inuence survival

rates of pale and melanic varieties.